As an electrophotographic color printer, a tandem electrophotographic color printer in which photosensitive drums corresponding to yellow, magenta, cyan, and black colors, respectively, are arranged in parallel with each other is known.
The tandem color printer is provided with a developing device which holds a developing roller so as to oppose each photosensitive drum. An electrostatic latent image is formed on the surface of the photosensitive drum. When the electrostatic latent image opposes the developing roller with the rotation of the photosensitive drum, toner is supplied to the electrostatic latent image from the developing roller, so that a toner image is formed on the surface of the photosensitive drum. Specifically, for color image formation, toner images of colors corresponding to the photosensitive drums, respectively, are formed, and the toner images of the respective colors are superposed on and transferred to a sheet conveyed by a belt. For monochrome image formation, only a black toner image is formed on a photosensitive drum for black, and the black toner image is transferred to a sheet.
At monochrome image formation, since any toner image is not formed on the photosensitive drums for yellow, magenta, and cyan other than black, it is advantageous to separate developing rollers from the photosensitive drums to reduce or prevent the wear of the developing rollers.
A tandem image forming apparatus includes a translation cam member which is linearly movable in an array direction of photosensitive drums, and an intermediate member which is displaced by the linear movement of the translation cam member, and presses a developing device upward to separate a photosensitive drum from a developing roller. Specifically, the linear movement of the translation cam member realize switching among an all-color separation state where developing rollers are separated from all the photosensitive drums, a black contact state where a developing roller contacts a photosensitive drum for black, and other developing rollers are separated from photosensitive drums for yellow, magenta, and cyan, respectively, and an all-color contact state where the developing rollers contact all the photosensitive drums, respectively.
Additionally, in the tandem image forming apparatus, the photosensitive drums for four colors are collectively held in a drum unit, and the drum unit is detachably mounted in an apparatus body. Further, developing devices for respective colors are detachably mounted in the drum unit.
However, in the above-described tandem image forming apparatus, when a developing device is pressed upward by the intermediate member and the developing device is moved, an upward force acts even on the drum unit with respect to the apparatus body due to the friction or the like between the developing roller and the drum unit. Accordingly, it is possible that the position of the drum unit with respect to the apparatus body deviates. If the position of the drum unit deviates, the position of an image to be formed on a sheet also deviates.